I played on 'grassy flatlands' and the BAI played much better on that, I also knocked up the difficulty level. Very good.
It was as I suspected, in the smaller, narrower maps when they have vast amount of units descending on one location they just get bunched up. On grassy flatlands they seemed to spread out a little more . They still melee'd my flanks, but that's to be expected, afterall in the real WW1 they would cross no man's land to get into the enemy's trenches. But they stood and fought with rifles a lot more too in the center.
Not sure if you've done it, but if I remember right, setting their attack parameters for 'rear' to 0 helped a lot, because giving instructions to go for the rear gets them confused and had them bunch up on flanks. But by removing rear and just increasing flank just slightly higher than center attack they understood that a lot better, and would naturally attack from the rear once around the flank anyway.
But yeah, so far BAI is looking good if you use a map with plenty of space to accommodate the unit numbers
nice job. fun battle that was!